Titanium base alloy



Nov. 14,196] J. B. McANDREW TITANIUM BASE ALLOY Filed Nov. 25, 1955 ATOMIC PERCENT ALUMINUM 55325035 0 w m 0 w w w w 0 0 0 w 0 2 O 8 6 4 2 O O O O 2 O O 3 3 2 2 2 2 2 w M M l m m I uv I 0 M l 9 T C A l m L T T 6 l L Y o L A E O E V 0'' M 4 M I 8 T 3 Q I M 13 2; 1 m m T I I o 5 m w Y I 4 M 4 o 5 5 ||||5||| 3 m 5 0 M. 0 2 ml 5 8 8 O O O O O O O O O O O O o O o o o O o o O o o o .b H mm .b M B W H m 9 7 6 u, musk/E523 v INVENTOR. JOSEPH B. MEANDREW TTORNEYS Sm tes Patent ce 3,008,823 Patented Nov; 14, 1961 3,008,823 TITANIUM BASE ALLOY Joseph B. McAndrew, Chicago, Ill., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Air Force Filed Nov. 23, 1955, Ser. No. 548,596 2 Claims. (Cl. 75--175.5)

The present invention is directed to improved titanium base alloys having excellent characteristics of strength, enhanced resistance to high temperatures, and excellent resistance to oxidation.

One of the biggest problems in the design of jet engines, rockets, and similar devices is the provision of a metal or alloy suitable for use in the extremely high temperature environment intended. Designers of jet engines, for example, realize that the efiiciency as well as the maximum speed of the engine could be increased if operation at higher temperatures could be tolerated. A metal or alloy selected for use in such engines, particularly as material for the manufacture of turbine blading and the like must have several properties which are not found in any single metal nor in most alloys. These properties include (1) good strength characteristics, even at high temperatures (2) resistance to creep at high temperatures (3) a low density, to reduce the inertia of the moving parts, and consequently the stress which the parts must withstand, and (4) satisfactory resistance to the extreme conditions of oxidative corrosion which occur in the jet turbine engines and the like.

In recent years, much emphasis has been placed on the use of titanium metal and alloys of titanium as suitable materials for this purpose. While titanium metal and titanium base alloys have a reasonably low density and good strength characteristics, they are frequently deficient with respect to creep resistance and in particular with respect to resistance to oxidation.

Accordingly, an object of the present invention is to provide an improved titanium base alloy having a useful combination of properties, including low density, good strength and creep properties at high temperatures, and excellent resistance to oxidation.

Another object of the invention is to provide a material having the characteristics given above and in which such characteristics are achieved without loss of the other desirable characteristics such as machinability, low density, and microstructural stability.

Still another object of the present invention is to provide a titanium base alloy particularly suitable for the manufacture of aircraft structural material, or in gas turbine engines. I

It has now been found that alloys fulfilling all of the above noted requirements can be produced from titaniumaluminum alloys of a specific portion of titanium-aluminum phase diagram with the inclusion therein of certain controlled amounts of the metals niobium and/or tantalum. More specifically, the alloy compositions of the invention can be described generally as gamma phase titanium-aluminum alloys containing amounts of tantalum, niobium, or mixtures of the two metals, in a weight range of 2 to 8% of the alloy. While the invention is particularly applicable to a ternary system consisting of titanium, aluminum, and one of the selected metals, substantial improvements in the physical properties of the titaniumaluminum alloys can also be achieved in quaternary alloys containing beside the three listed metals, elements such as silver or nitrogen. In all cases, however, the quaternary alloying agents are present in only minor amounts, and the total content of aluminum and titanium is at least 90% by weight.

The preferred alloy according to the present invention is one containing from about 35 to 36% by weight aluminum and from about 5 to about 7% by weight of niobium, tantalum or mixtures of the tWo, with the balance of the alloy being substantially all titanium, with the usual minor amounts of impurities.

It has been found that the addition of niobium and tantalum provides substantially improved oxidation resistance in alloys comprising the gamma intermediate phases of the binary and polynary titanium-aluminum systems. Alloys of this type are inherently resistant to penetration by diffusion of oxidizing elements in the metallic phase itself. The improvement in oxidation resistance is therefore largely the result of an improvement in the physical and chemical properties of the oxidized material which forms as a coating when the alloys are subjected to high temperature conditions of oxidation. When the binary alloy consisting of 36% aluminum and the balance titanium is oxidized in air, an X-ray analysis indicates that the coating which results is a mixture of titanium dioxide and alpha alumina, with no evidence of the presence of nitrides. The presence of niobium and/ or tantalum in the alloy results in the formation of non-volatile, bulky oxides which apparently improve the oxidation resistance by decreasing the density of the oxide mixture as compared to that of the underlying metal. The oxide coating therefore forms under compressive forces which prevent cracking and reduce the permeability of the coating and to some extent reduce the mobility of the constituents.

An understanding of the invention will be facilitated by reference to the accompanying drawing which represents the titanium-aluminum phase diagram.

As indicated in this diagram, the gamma phase is the predominant phase in the titanium-aluminum system where the aluminum content is between about 3 0 and 53 by weight, when the structure is in equilibrium below 1000 C.

The titaninum employed in the manufacture of the alloy specimens was a titanium sponge having a hardness of 121 Brinell using a 3000 kilogram load.

The impurity analysis of the titanium sponge was as follows:

Percent C 0.058

Si 0.018 Fe 0.068 N max 0.020

The aluminum employed in the manufacture of the alloys was in shot form with the following analysis:

The alloys were manufactured in an electric casting furnace using a non-consumable electrode consisting of a water cooled, tungsten tipped electrode. The melting of the ingredients was accomplished under vacuum conditions or in the presence of an inert atmosphere to avoid contamination by oxygen or nitrogen.

The oxidation tests carried out on the various samples were made on cylinders /2" long x 0.357" in diameter and cuboid pieces of about size. The samples were measured with a micrometer calipers, placed on ceramic trays, and exposed in electric furnaces of either the tube or the muffle type. Openings of /2" in diameter in the furnace permitted access of air. Temperatures at the specimens were checked initially with a thermocouple and potentiometer and in longer tests additional checks were 3 made from time to time to insure that the controller thermocouple properly reflected the actual specimen temperature. After exposure, the oxide film was carefully ground away and the specimens were remeasured to determine the depth of penetration of oxidation.

Two factors permit the use of this method to check oxidation. First, the alloys under investigation have not shown the troublesome effects encountered in other titanium alloys due to difiusion of oxygen or nitrogen into the metal in advance of the oxide or nitride interface. Second, the specimens in most instances showed an absence of pitting.

The following table indicates the results achieved in oxidation tests performed as described previously with various alloys of the present invention:

136 hrS., 1,000 137 hrs., 1,200

In comparison to the above, an alloy containing 36% aluminum and the balance titanium evidenced a depth of oxidized metal, for a period of 100 hours at 1000 C. in still air of 0.012 inch.

When silver and nitrogen are employed in the alloy compositions, it is desirable to keep the silver content not in excess of about by weight, and the nitrogen content not in excess of about 1% by weight.

Tensile strength tests were also made on numerous samples, and the results indicated that the addition of the controlled amounts of niobium and tantalum also serves to increase the ultimate tensile strength of the alloys significantly. The following table compares the ultimate tensile of the improved alloys of the present invention as compared with a typical gamma phase alloy of titanium and aluminum:

Composition, Wt. Percent Ultimate Tensile Strength, p.s.i.

Ti-36A1 40, 000 Ti36Al2.5Nb 42, 260 Ti35Al5Nb 62, 000 Ti35Al5Ta 46, 600 Ti36A17Ta 1 71, 060

4- microstructural stability. The improved properties obtained permit the use of the improved alloys at higher temperatures, or under higher unit stresses at a given temperature, than can be withstood by the base alloy.

A single stress-rupture test of an alloy containing 35% aluminum, 7% niobium, and the balance titanium also showed greater strength at 950 C. than that of the binary alloy containing 36% aluminum and the balance titanium. The niobium containing alloy was stressed to 14,000 p.s.i. and failed at 135 hours. Under the same conditions, and for the same time of failure, the base alloy consisting of 36% aluminum and the balance titanium could only be stressed to 9,200 p.s.i.

The alloys of the present invention, unlike many refractory materials, can be prepared by casting with the use of conventional methods which have been successfully employed on other titanium base alloys. In order to insure a homogeneous alloy, however, it is desirable to use master alloys of titanium and niobium such as one containing 25% niobium and titanium, or a 50-50 alloy of 'titanium and tantalum to introduce these elements into the alloy. In the preparation of the alloy, care should be taken to avoid contamination with hydrogen as the presence of this element can cause porosity of the resulting casting, which is true with titanium-aluminum gamma phase alloys in general.

From the foregoing, it will be apparent that the alloys of the present invention are particularly adapted to high temperature use under conditions of oxidative corrosion. It will also be evident that various modifications can be made to the described embodiments without departing from the scope of the present invention.

I claim as my invention:

1. An alloy composition having improved high temperature properties and improved oxidation resistance properties comprising a titanium-aluminum alloy containing from 35% to 36% by weight aluminum, from 2 to 8% by weight of a metal selected from the group consisting of tantalum, niobium, and mixtures of the two, with the balance of said alloy being substantially all titanium.

2. An alloy composition having improved high temperature properties and improved oxidation resistance properties comprising from 35 to 36% by weight aluminum, from 5 to 7% by weight of a metal selected from the group consisting of tantalum, niobium, and mixtures of the two, with the balance of said alloy being substantially all titanium.

References tlited in the file of this patent UNITED STATES PATENTS 2,754,204 Iaffee et al July 10, 1956 FOREIGN PATENTS 726,203 Great Britain Mar. 16, 1955 718,822 Germany Mar. 24, 1 942 

1. AN ALLOY COMPOSITION HAVING IMPROVED HIGH TEMPERATURE PROPERTIES AND IMPROVED OXIDATION RESISTANCE PROPERTIES COMPRISING A TITANIUM-ALUMINUM ALLOY CONTAINING FROM 35% TO 36% BY WEIGHT ALUMINUM, FROM 2 TO 8% BY WEIGHT OF A METAL SELECTED FROM THE GROUP CONSISTING OF TANTALUM, NIOBIUM, AND MIXTURES OF THE TWO, WITH THE BALANCE OF SAID ALLOY BEING SUBSTANTIALLY ALL TITANIUM. 